in which our heroine acts like a real writer

January 13, 2011

As a part of applying to the AAAS Mass Media Science Fellowship, I was required to write a 750-word news story about a technical journal article. I had a blast doing it, and it was an interesting opportunity to tackle an article that made a splash in my field back in October, which I was especially keen to do since the media that picked it up did such an abysmal job with it. After writing this, I definitely have a little more of an idea why scientists hate science journalism so much, and why it’s not 100% fair to science journalists to trash them for it. To break complex research down in a limited amount of space, a writer has to leave a lot of information out, and some of it will no doubt be important information. In the article below, I had to ditch a few concepts that are absolutely crucial, and it killed me to do it.

At any rate, at the end of this exercise, I was left with what I think is a decently well-written article about an important achievement in my field. What’s a girl to do but share it with her loyal blog stalkers, many of whom would really like to hear more about my research (research related to it, in this case)? With no further ado….

Solar energy breakthrough: two for the price of one

Solar technology is a cornerstone of our transition to renewable energy, but traditional solar cells can never be more than about 30% efficient. New solar innovations may one day break that limit, and now a team of researchers based at the University of Wyoming has demonstrated one way that it might be done.

All solar cells operate on the same basic principle: they convert energy from sunlight into electricity. By putting a little bit of light in, we get a little bit of electricity out. It turns out that “a little bit” has an exact meaning here: both light and electricity are “quantized,” meaning they only come in specific amounts, like packages at the store. In traditional solar cells, we can use one package of light – one photon – to buy one package of electricity – one electron. This price is what ultimately sets the upper limit of 30% efficiency.

One way researchers can beat that limit is to try to change the price, which means changing the material used. In strange materials known as “quantum dots,” electricity is buy one get one free: one photon of light can buy two electrons of electricity. Unfortunately, there’s a catch: there has never been a way to extract those electrons. We can get electrons for cheap, but we can only take advantage of the deal if we never leave the store with our purchase.

In a new study published in Science, a group led by Bruce Parkinson at the University of Wyoming has found a way to change that. His team has developed a quantum dot solar cell where one photon of light can produce two electrons of electricity, and for the first time, that electricity can be collected and used outside of the cell.

Their solar cell is radically different from the familiar rooftop devices we see today, which consist of a single slice of solar material – usually silicon – connected to metal electrical leads. Instead, Parkinson’s team uses a thin slice of titanium dioxide as an electrical contact, and applies only a thin coating of the solar material, lead sulfide quantum dots. Sunlight is absorbed and converted into electricity in the quantum dots, and the electricity is then extracted through the titanium dioxide.

This solar cell design is actually nothing new. Known as “sensitized solar cells,” these devices originally used a coating of molecular dye to convert sunlight to electricity. Because good solar materials tend to be expensive, a sensitized solar cell can reduce costs by using less material than a more traditional solar cell. They also require much less energy to produce, and can be manufactured into lightweight and flexible sheets.

One of the most attractive things of all about sensitized solar cells is that the design makes it possible to separate the jobs of converting sunlight and of conducting electricity. A traditional solar cell needs a material that is a good converter and a good conductor, and these two properties are almost always in conflict. A sensitized solar cell, however, can use one material to do the conversion, and a different material to do the conduction, taking advantage of the best properties of each.

The trick to making a good sensitized cell lies in building the connection between the converting material and the conducting material. The converting material does all the real work of the solar cell, but without a good connection to the conducting material, the generated electricity can’t go anywhere and will eventually dissipate as heat. This has long been the problem for quantum dot sensitized solar cells: quantum dots offered buy one get one free prices on electricity, but lacked a good connection to the conducting material, meaning none of that cheap electricity could leave the store.

Now, Parkinson’s team has discovered how to build a good connection between lead sulfide quantum dots and titanium dioxide contacts. Using a chemical called mercaptaprionic acid as a bridge, electrons in their solar cell can cross efficiently between the lead sulfide quantum dots where they are produced into the titanium dioxide where they are collected.

Unfortunately, despite the remarkable efficiency of the quantum dots themselves, other problems make these cells less efficient overall than traditional solar cells. Huge challenges remain before the technology can be scaled up for mass production, and quantum dot sensitized solar cells have a long road ahead before they can compete with traditional technology. Regardless, Parkinson’s team has taken an important first step.